A coupled diffusion–degradation framework for hygrothermal service-life prediction of GFRP laminates in hydroelectric applications

Abstract

GFRP laminates used in water-immersed hydroelectric applications, such as turbine stay vane extensions, are prone to hygrothermal-induced mechanical degradation. This paper develops and validates a one-way coupled diffusion–degradation framework for six laminate configurations to predict long-term hygrothermal performance as a function of matrix type (epoxy and vinylester), reinforcement architecture, and surface veil presence. Specimens were immersed at ambient (∼23°C) and accelerated (40°C) conditions. All systems exhibited Fickian diffusion behavior; however, equilibrium moisture content differed markedly between epoxy and vinylester laminates ( $M_m$ ≈ 5.8% and 2.9%), with both values largely independent of reinforcement architecture. A polyester surface veil reduced the effective diffusion coefficient by approximately 50% without affecting $M_m$ , thereby delaying moisture ingress into the structural core. An Arrhenius–Phillips framework was applied to tensile property data, revealing that mechanical degradation is diffusion-controlled, as the activation energy for strength degradation (≈50–60 kJ/mol) is consistent with that for moisture transport. Among the configurations investigated, the vinylester/NCF/veil laminate (VNV) exhibited the highest hygrothermal stability; Arrhenius-shifted projections to river-water temperatures (5–15°C) predict tensile strength retention exceeding 90% after 50 years of continuous immersion. The proposed framework provides a physically grounded and experimentally validated methodology for hygrothermal service-life prediction of water-immersed GFRP components, with direct applicability to hydroelectric turbine design and rehabilitation.